The present invention relates to a shaking table for use as a shaking test apparatus and a method of controlling the same, and more particularly to a shaking table having improved shaking acceleration reproducibility and an improved control method.
A shaking table is an apparatus used for the shaking test of a structure or an apparatus for shaking a structure by driving a table on which the structure as a specimen under test is placed by an actuator. The table may be held by proper bearings.
One of the application fields of the shaking table is the seismic test, in which the earthquake resistance of a structure is tested. Because the purpose of the seismic test is to evaluate the response of a structure to predetermined acceleration waveforms, it is required to reproduce with accuracy the predetermined acceleration waveforms using the table. Therefore, increasing the accuracy of such an acceleration waveform resulting from shaking is obtainable by installing displacement and/or acceleration sensors on the shaking table in order to measure the motion of the table and feed back the measured results to a shaker control unit with predetermined acceleration waveforms as target values (e.g., "Servo Mechanism of Three Dimensional Six Freedom Shaking Table" by Sugano, Machine Design (Japanese), Vol. 35, No. 4, April, 1991, pp 25-28).
In a case where a specimen under test put on the table is relatively light, those methods mentioned above may be used to obtain highly accurate waveforms resulting from shaking, but when the weight of a specimen under test is heavy, desired acceleration waveforms cannot be obtained because the force from the specimen under test generated by shaking and applied to the table is also large. Because the counter-reaction force from the specimen under test acts to restrict the motion of the table particularly when the specimen under test is shaken at a frequency close to the natural frequency of the specimen under test, the acceleration waveform resulting from shaking tends to become small.
As disclosed in "Electro-Hydraulic Shaking Machine (3rd Report)" by Hirai and Matsuzaki in Bulletin of Japanese Society of Mechanical Engineers (Japanese)., Vol. 42, No. 361, pp 2,744-2,751, techniques for improving acceleration waveforms resulting from shaking have been proposed by installing models of specimens under test in the control circuits of shakers as compensators. As also disclosed in Japanese Patent Laid-Open No. 197902/1984, a random wave having a relatively small amplitude is input as a target acceleration waveform in such a state that a specimen under test has been loaded, an actually obtained accelerator waveform resulting from shaking a transfer function from the target to the actually obtained acceleration waveform resulting from shaking is calculated, and at the time of actual shaking the target waveform via the inverse transfer function is used as the input.
This known technique is effective when the natural frequency and/or damping is constant and the dynamic characteristics of the specimen under test remain unchanging during the shaking and the reproducibility of acceleration becomes good. When the specimen under test has non-linearity, however, the characteristics may vary during the shaking because of the dependence on the response amplitude. In a case where the specimen under test is shaken until it is damaged, the stiffness of the specimen under test rapidly changes, thus causing the dynamic characteristics to vary. In the case of a shaking test on the ground liquefaction, the occurrence of liquefaction makes the dynamic characteristics change greatly. In this case, the acceleration waveforms obtained through the aforementioned methods may become different from those desired.
Even in fields other than seismic testing, it may become necessary to evaluate strength by shaking a structure with acceleration of the random waveform having a predetermined spectrum density. In this case, such a predetermined spectrum density may not be obtained due to a phenomenon similar to the case of the earthquake test. In order to solve this problem, various methods including the aforementioned ones have been employed; nevertheless, shaking accuracy may be lowered due to the non-linearity of the specimen under test.
An object of the present invention for solving the foregoing problems is to provide a shaking table making available highly accurate acceleration waveforms resulting from shaking without being influenced by a specimen under test on a table and by changes of the dynamic characteristics of the specimen, and a method of controlling the shaking table.
A shaking table according to the present invention comprises a table for loading a specimen under test, driving means for driving the table, (first measuring means for measuring the acceleration of the table, and a control unit for for receiving a target signal and comparing the target signal with an output from the first measuring means in order to effect the feedback control of the driving means on the basis of a deviation from the target signal wherein the table may be provided with second measuring means for detecting the force applied from the specimen under test and wherein the output from the second measuring means may be fed back to the control unit.
At this time, the control unit may be provided with a filter having for its transfer function the inverse transfer function of a transfer function from a position where the force detected by the force measuring means is fed back up a position where the driving force of the table is generated. In one preferred mode, a transfer function up to the driving means for driving the table may be taken into consideration.
Such a filter may be provided as hardware means such as an electric circuit or implemented by calculating means such as a DSP (Digital Signal Processor) When this filter is implemented by the provision of the calculating means, it may be provided for the control unit.
For the shaking table according to the present invention, the force measuring means may be provided between the table and the specimen under test.
For the shaking table according to the present invention, the force measuring means may be provided between the driving means and the table.
For the shaking table according to the present invention, an acceleration detector as the first measuring means, and processing means for subtracting a value resulting from multiplying the acceleration detected by the acceleration detector by the mass of the table from the force detected by the force measuring means may be provided.
A shaking table according to the present invention comprises a table for loading a specimen under test, driving means for driving the table, first measuring means for measuring the motion of the table, and a control unit for feeding back the value detected by the first measuring means and comparing the feedback value with a target value in order to control the driving means on the basis of a deviation from the target value. An oil-hydraulic actuator as the driving means, means for detecting the differential pressure in the piston portion of the oil-hydraulic actuator, processing means for multiplying the differential pressure by the area presented to the piston portion, and means for correcting the deviation on the basis of the result processed by the processing means may be added to the construction of the shaking table.
A method of controlling a shaking table according to the present invention comprising a table for loading a specimen under test, driving means for driving the table, first measuring means for measuring the motion of the table, and a control unit for feeding back the value detected by the first measuring means and comparing the feedback value with a target value in order to control the driving means on the basis of a deviation from the target value includes the steps of measuring the force applied to the table from the specimen under test, subjecting the force to filter processing, and processing the subtraction of the processed result from a target acceleration waveform in order to control the driving means on the basis of this processed result.
At this time, the filter processing may be performed with the inverse transfer function of the transfer function from a subtraction processing unit for subtracting the result of the filter processing from the target acceleration waveform up to the driving means.
A shaking table according to the present invention comprises a table for loading a specimen under test placed on a base via bearings, an actuator for shaking the table fixed onto the base, an acceleration detector for detecting the acceleration of the table, a load cell placed on the table, for detecting the force applied to the table from the specimen under test while carrying the specimen under test and a control unit for controlling the actuator on the basis of the force detected by the load cell and the acceleration. The control unit may include means for correcting the force, comparison means for obtaining the deviation of the corrected force from a target value, means for amplifying the deviation, and means for inputting the amplified deviation to the actuator, wherein the means for correcting the load may be adapted for correcting the force by using the inverse transfer function of a transfer function from the comparison means up to the shaker.
A shaking table according to the present invention comprises a table for loading a specimen under test placed on a base via bearings, an oil-hydraulic actuator for shaking the table fixed onto the base, an acceleration detector for detecting the acceleration of the table, means for detecting the differential pressure in the piston portion of the oil-hydraulic actuator, and a control unit for controlling the oil-hydraulic actuator. The control unit may be provided with means for obtaining a force by multiplying the differential pressure by the area presented to the piston portion of the oil-hydraulic actuator, means for correcting the force, comparison means for obtaining the deviation of the corrected force from a target value, means for amplifying the deviation, and means for inputting the amplified deviation to the actuator, wherein means for correcting the force may be adapted for correcting the force by using the inverse transfer function of a transfer function from the comparison means up to the actuator and wherein the control unit may control the oil-hydraulic actuator on the basis of the differential pressure and the acceleration.
According to the present invention, because the force applied to the table from the specimen under test can be canceled by the driving force applied to the table from the driving means, acceleration or an acceleration waveform is highly accurately reproducible without being affected by the fluctuation of the dynamic characteristics of the specimen under test.